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I was just illustrating the only scenario where working hard doesn't pay off. Milestone exams SATs, GCSEs and A-levels aren't so easy as to make hard work pointless. The harder you work the better you will do. However, if you do cram and do well then you performed well, are not a 'chancer' and deserve a good grade just as much as someone who spent four times as long revising for it.
E: I've realised I'm trying to pat myself 15 year old self on the head for getting C's and U's in my GCSE mocks.
If the diligent ones don't outperform the 'chancers' then they don't get as good a grade yes, but if they can't manage that then they aren't brighter than the chancers are they? That's the point of the exam to test performance not effort.
If the exam is challenging enough to require more work, then longer term effort will be rewarded over cramming.
Similarly, if the exam is so easy that you can get a good mark with a few hours revision then you'd be pretty dumb to spend half the term preparing for it.